Interior monologues, the fancy literary term for a character's thoughts in a novel, are another device used in effective writing. Whether long or short—after a scene or peppered throughout—many beginning novel writers want to know specifically how to render a character's thoughts on the printed page. Should they italicize, for instance, or include a "he thought" tag?
You have a few options when it comes to formatting your interior monologues to indicate that it is in fact the character's thoughts and not the narrator narrating:
Italics are used to represent a character's thoughts as they actually think them in their head. For that reason, italicized thoughts are always written in the first person because we don't think about ourselves in the third person and present tense because we don't think about what is happening in the here and now in the past tense. The benefit of using italics for an internal monologue is that they make the thought stand out. It becomes obvious to the reader that these words are the character thinking and not the author narrating. And the thought itself, as well as not becoming confused with the rest of the text, gains extra emphasis.
The trouble with italics, however, is that they can be irksome to read. If you use italics for every single thought in the novel, not just the one-liner thoughts but the longer ones that might run on for several paragraphs or pages, the reader won't thank you for it. Also, because italicizing thoughts gives the words much greater emphasis, the effect you create could turn out to be inadvertently laughable.
The solution, if you want to use italicized thoughts, is to use them parsimoniously. Only write a character's most imperative and moving thoughts in this way. For the rest, use a more elusive manner of presenting internal monologue like non-italicized. The reader will quickly pick up on this convention if you use it consistently. What they would find puzzling would be if you used, for instance, an inconsistent jumble of italics and non-italics for those intermittent emphatic thoughts.
Thought Tags are an alternative to using italics. Like dialogue tags, their only real purpose is to make it clear to the reader that these are the character's thoughts and not the narrator's words. If it is clear without using a tag - either in monologue or dialogue - don't use one. First person point of view, for example, typically does not require thought tags because the “camera” is always behind the character's eyes, and so it is apparent when we hear their direct thoughts.
However, if a thought is written in the third person it may or may not be suitable to use a tag. It all depends on where the "camera" is situated. When the narration is more distant than close you will probably want to use a "he thought" tag. When the narration is close and intimate, and the language is beginning to take on the viewpoint character's own speaking voice, tags won't be needed. Interior monologue thought tags usually appear during the cooler beginnings of scenes and not after they have warmed up.
The Character's Natural Speaking Voice. In a first person novel, you hear the viewpoint character's natural speaking voice openly. In a third person novel, you only hear their direct natural voice in dialogue or in monologue rendered in the first person. For the rest of the time, you hear the narrator's voice, which is less biased, less flamboyant, less colloquial than the viewpoint character's voice. During the cooler, opening section of a scene, any lines of internal monologue are best written fairly neutrally and factually—and they should probably be tagged, too. Once the scene has heated up, the internal monologue (while remaining in the third person) can begin to adopt the individualities of the character's natural first person voice.
Examples of all of the above can be found in published fiction, so in a sense it is a matter of personal choice. The only hard and fast rule that exists is to be consistent throughout. Readers quickly grow accustomed to whatever conventions you have decided to use, and not adhering to those conventions steadily will only confuse your audience.